function reflection
E0=1;
phi=pi/2;
z=linspace(-15,0,1000);
zt=linspace(0,15,1000);
y=zeros(1,1000);
w=pi/2;
t=0;
Ei=E0.*cos(w.*t-phi.*z);
Er=-E0.*cos(w.*t+phi.*z);
plot3(z,y,Ei);
hold on
plot3(z,y,Er);
hold on
xlabel('Z');ylabel('y' );zlabel('x');
